Ecological Factors Influencing Pea Aphid Outbreaks in the US Pacific Northwest

This chapter documents the history of pea aphid (Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae)) outbreaks in the U.S. Pacific Northwest, with particular attention to the periodicity of outbreaks on field peas (Pisum sativum L.) in the Palouse region of eastern Washington over a 26 year period (1983–2008). In the Palouse region, grain legume crops are devastated by pea aphid feeding damage and legume viruses during outbreak years. Various abiotic and biotic factors and their possible controlling influence on changes in pea aphid densities, with emphasis on winter temperatures within the context of climate change, are presented and discussed. Mild winters, long thought to herald spring pea aphid outbreaks, are defined and related to outbreak cycles. While the 26-year database does not demonstrate a consistent link between mild winters and pea aphid outbreaks, it reveals a certain periodicity with outbreaks occurring every 6–9 years in the Palouse region. Warming winter temperatures over several years bridging the 20th and 21st centuries could potentially compress the frequency of outbreaks by accelerating aphid population development on legumes that provide migrants that colonize peas and other spring-sown food legumes in this region. Large plantings of alfalfa (Medicago sativa L.) 60–120 km west and southwest of the Palouse region are thought to be the source of alate aphids that migrate northeast and attain outbreak densities in some years. In conclusion, this chapter delivers a new long-term data set that will improve our understanding of how different abiotic and biotic factors influence the life-history processes of pest aphids.